Trucking companies typically operate both dump trailers and live floor trailers. Dump trailers utilize single acting single rod end telescopic cylinders that extend to raise the dump body and then retract to return the dump body to its transit position. The hydraulic system for dump trailers has a directional control valve integrated into the port end cover of the hydraulic pump used to actuate the telescopic cylinders. When the dump body is in the raise position, pump flow to the tank is blocked and the flow is directed through the load check valve to the hydraulic cylinder. In the neutral position, pump flow to the tank is allowed while the load check valve locks the hydraulic cylinder. When the dump body is in the lower position, both the pump flow and the cylinder flow vent to the tank. A relief valve protects the hydraulic cylinder in both the raise and neutral positions. Dump trailers typically require a flow of 30 to 35 gpm at 2,000 psi.
Live floor trailers have hydraulically powered groups of aluminum floor slats that extend along the length of the trailer. The groups of floor slats move in a four phase cycle. In the first phase of the cycle, the first group of three slats moves to the front of the trailer and the load does not move. Then the second group of three slats moves to the front of the trailer followed by the third group of three slats. Again the load does not move. In the fourth phase of the cycle, all of the slats move to the rear of the trailer and the load moves rearwardly. Live floor trailers require a flow of 30 to 35 gpm at 3,000 psi.
From the foregoing, it is obvious that trucking companies encounter a problem when they operate both dump trailers and live floor trailers since different operating pressures and thus, separate relief valves are required for each application. A truck with a hydraulic system that is set up for dump trailers cannot haul live floor trailers and a truck that is set up for live floor trailers cannot haul dump trailers. Because of this, trucking companies are limited in the use of their assets.
In view of the foregoing, it has become desirable to design a multiple pressure relief valve that allows the hydraulic pump to operate at two adjustable actuating pressures permitting the relief valve and associated circuitry to be operable for both dump trailer and live floor trailer applications.